In passing through the human body, gamma photons have a certain probability of scattering due to the Compton effect. Such scattering changes the direction and energy of the photons. When a photon that has been scattered is detected by the gamma camera, false position information is derived from the scattered photons. Thus, the scattered photons cause events that are unwanted for use in constructing the image. Other unwanted events exist. For example, the radiation emitted from the patient often excites lead (K) X-rays from the collimator and other lead parts. These X-rays also impinge on the detector and may be registered as events. These X-ray photons constitute an additional source of image blurring.
The problem of X-ray induced events arises especially for radio isotopes emitting photons in the energy range of 88-120 KEV. In this range, the lead X-ray excitation probability is high and the spectrum of these photons coincides with a relevant part of the isotope's spectrum, but partially overlapping the photopeak. Thus, the unwanted part of the spectrum in each pixel has two terms: one made up of the Compton scattered photons, and the other made up of the lead X-ray photons.
In principle all of the events caused by unwanted photons should be discarded. However, it is not easy to arrive at criteria that are efficient and effective for discarding such events. For example, an energy level criterion is not effective because although the photon loses part of its energy in the scattering process, the energy resolution of the typical gamma camera is such that there is a large amount of overlap between the energy of unscattered and scattered photons.
The invention of the previously mentioned patent application provided methods and means for qualitatively and quantitatively improving the recorded images by significantly reducing the contribution of Compton scattered and other unwanted photons to the final image thereby providing an image practically free of unwanted photons within seconds after acquisition. The inventions accomplish the task of reducing the number of events caused by unwanted photons by locally determining the energy spectrum and fitting this spectrum with a "trial" function composed of a photopeak component, a set of components representing various orders of Compton scatter and, if necessary other unwanted photons, the instrumental energy dependance of all of which having been calculated or experimentally determined. The fitting procedure determines the local amplitude of the photopeak component and, if required, also the amplitudes of the other above mentioned components.
The true physical characteristics of the Compton process are used in the previously mentioned Patent Applications to derive Compton multi-scatter functions which are subsequently used to construct the Compton scatter component energy spectra. Thus, the previous Patent Applications use the following inputs to determine the unknowns; (i.e., the magnitude of the photopeak component and the magnitude of the Compton multi-scatter components):
1. the measured energy spectrum per pixel. This includes counts due to scattered and unscattered photons, and PA1 2. the measured system energy spread function for the isotope centerline which provides the photopeak energy shape. PA1 detecting photons impinging on a gamma ray detector as event counts, PA1 determining an X, Y location for each photon according to the location of the impingement of the photons on the detector, PA1 measuring the energy of said impinging photons, and PA1 grouping each detected photon according to the measured energy and the X, Y location, PA1 accumulating counts of said photons at each X, Y location according to the determined energy level of the photons, PA1 constructing a measured energy spectrum at each X, Y location using the accumulated counts of the determined energy levels, said measured energy spectrum including counts of wanted and unwanted photons, PA1 determining the energy distributions of unwanted photons, PA1 determining the energy spread function of the gamma ray imaging system being used, PA1 obtaining a system dependent energy distribution of the unwanted photons per location by using the energy distribution of the unwanted photons and the energy spread function of the system, PA1 constructing a trial function comprising the system dependent energy spread function multiplied by an unknown coefficient of wanted photons plus unknown coefficients of unwanted photons convolved with the system's energy spread function, PA1 said trial function further including the derivative of the energy spread function of the wanted photons multiplied by the unknown coefficient, PA1 solving for the unknown coefficient of the wanted photons by locally fitting the measured energy distribution to the trial energy distribution of photons, and PA1 using the count of the wanted photons to produce an image practically free of unwanted photons and free of artifacts caused by local effective deviation between the measured energy and the true energy.
The shape of the Compton component of the trial function is analytically derived in the first application by using the Nishina-Klein Equation that describes the physical relativistic scattering of photons with electrons to derive a probability distribution for a photon to scatter from a given energy to a lower energy in a single interaction with an electron. Repeated convolutions are used to obtain the probability distribution for the higher order scatter terms.
By locally fitting the trial function to the measured energy spectrum of acquired data, the photopeak magnitude and possibly the values of additional components are obtained. This enables the removal of contamination of unwanted photons from the acquired data.
The invention of the above mentioned first filed application, however, assumed a single photopeak. In certain isotopes there is more than one photopeak. If a single peak is assumed when more than one peak actually exists, the removal of scattered events from the image will be incomplete.
Accordingly, the other Application was filed. The invention of that Application is an improvement over the invention of the first mentioned Application in that, among other things, it takes into account radio isotopes having more than one peak and also takes into account all unwanted events due to Compton scattered photons and photons derived from such phenomena as X-rays caused by gamma radiation interacting with lead components.
It is an object of this present continuation-in-part application, among other things, to improve the stability of the fit of the measured energy spectrum per pixel (or super pixel) to the trial function vis-a-vis local residual errors in the measurement of energy. Local residual errors include those caused by "drift" of components such as photomultiplier tubes, amplifiers, etc. The phrase, "stability of the fit" for purposes of this application, means the proximity of the fitted parameters of trial function to the real parameters of incident photon flux. Pixels as used herein included single picture elements or groups of picture elements, i.e. super pixels.